The present invention is concerned with molding, specifically by a technique employing freezing and pressure, of inorganic powders, typically ceramics.
Inorganic materials, notably ceramics are recently establishing a position of importance as industrial materials second only to metals and plastics. Particularly in engineering applications, new ceramics and fine ceramics, because of their advantages over metallic materials in terms of lightness, greater hardness and wear resistance, and thermal resistance, are rapidly finding new markets and uses. Applications are anticipated as sliding components or components exposed to high temperatures.
To deliver adequate performance, such mechanical components must generally have a complex, configuration, plus high dimensional accuracy. Ceramics, however, because of their resistance to heat and wear, are a classically unsuited to machining, so that it is highly desirable that the final form should be provided at the time of molding by means of a ceramic powder molding technique.
One such technique is slip casting, which is used traditionally in the field of ceramic ware. This is in effect a casting method in which slip is poured into a plaster of Paris mold, which absorbs the water. It is, however, generally limited to the molding of comparatively simple shapes, and suffers from three disadvantages: (1) large numbers of plaster of Paris molds must be provided; (2) because the molds react with the water and begin to dissolve, mold life is extremely short; and (3) drying requires long periods of time, reducing productivity.
Injection molding is eagerly awaited as an alternative method of molding ceramic powders. If this method can be appropriately applied, it will become possible to produce ceramic products of complex configuration with a high degree of efficiency.
Ceramic powder by itself, however, is not fluid, so that molding it has traditionally required the admixture of large amounts of resin binder. Thus the product is actually plastic, with a high ceramic content.
This led to the following problems:
1. If dewaxing (the removal of the resin binder by heating and decomposing into gas prior to sintering) is carried out too precipitously, scaling and deformation occur. To avoid this, the rate of temperature rise must be reduced and high temperatures cannot be used. As a result, the process of removing the binder requires an inordinate amount of time (from 4 to 7 days), productivity is reduced, and vast amounts of heat are required, leading to higher production costs. PA0 2. If the resin binder is totally removed by this process, strength decreases, so that a portion of the binder must be left in place. After final sintering, the binder residue produces defects in terms of strength. PA0 3. When a resin binder is used, it is mixed with the ceramic powder, heated, and injected into the mold. Since, however, the viscous resistance of the binder is greater, the behavior of the binder when flowing gives rise to uneven distribution of the powder in the molded object, which tends to manifest itself after sintering as product defects. In places where the resin binder flows readily, the powder density is lessened, while it becomes correspondingly greater in the corners. Furthermore, the resin may be concentrated along the weld line (the flow front of the mixture) so that a resin binder layer is present on the surface leading to increased surface roughness after sintering. PA0 4. If the amount of resin binder is reduced, molding parameters such as the pressure and temperature of injection become more critical and harder to control.
Another feasible method might be compression molding using metal dies, but subsequent release from the die is then difficult, and using knockout or ejection pins will result in deformation. It is still necessary, therefore, to use large amounts of resin binder, taking advantage of its flowability and setting properties, and here again the same problems as in the case of injection molding are encountered.